The present invention relates to a stop-weld composition especially for forming a corrosion inhibiting layer on the internal surface of aluminium tube in sheet.
Throughout the specification and appended claims, the percentages used in connection with the composition are all by weight.
Methods are already known by which a plurality of aluminium or aluminium alloy sheets are pressure-welded together with a stop-weld composition sandwiched therebetween in the desired pattern and fluid pressure is thereafter applied to the unwelded portions to inflate or bulge the unwelded portions to a tubular shape. The product thus fabricated will be herein referred to as an "aluminium tube in sheet."
Conventionally known as the above-mentioned stop-weld composition are those predominantly consisting of colloidal graphite and those chiefly composed of titanium dioxide (see U.S. Pat. No. 3,385,716). Stop-weld compositions should be removed by cleaning after the bulging operation but are difficult to completely remove. Graphite, which is a strong cathodic depolarizer, causes electrochemical corrosion in contact with aluminium in the presence of water, so that the use of stop-weld compositions of the former type entails the corrosion of the inner surface of the tube. Although stop-weld compositions of the latter type comprise an electrochemically neutral substance free of such drawback, they are inferior to the former in the weld preventing effect and therefore involve frequent difficulties in the bulging operation.
Aluminium tube in sheets have already found a wide variety of applications in heat exchangers. When they are used for evaporators for refrigerators or the like in which the tube serves as a gas conduit, the internal surface of the tube is not subject to corrosion, whereas when they are applied to radiators, boilers for bath and like household uses and water heaters utilizing solar heat in which the tube serves to pass water containing copper, iron or like heavy metal dissolved therein, corrosion and corrosion pitting will invariably take place, because the electrode potential of aluminium is lower than that of such metal.
In view of the foregoing problems, a method for producing aluminium tube in sheet having a corrosion inhibiting layer on the internal surface of the tube was proposed (see U.S. Pat. No. 3,650,005) In this method, a stop-weld composition containing finely divided zinc is used to form a corrosion inhibiting layer on the internal surface of the tube. However, the stop-weld composition, which also contains graphite, similary poses a corrosion problem owing to the graphite remaining after cleaning.